Method for the production of fatty acid esters

ABSTRACT

A method for the production of fatty acid esters of primary and/or secondary alcohols from raw, unpurified oils or fats containing free fatty acids and slimy substances or from waste oils loaded with free fatty acids from the food industry. Acidic catalytic esterification is performed on the free fatty acids along with extensive deactivation of the slimy substances present with respect to the disadavantageous emulsion-forming property thereof, whereby a water-free primary and/or secondary alcohol and a highly acidic hygroscopic catalyst are added to the starting substance in the form of crude oil or fat or waste oil, the reaction product is rinsed with glycol from a prior alkaline transesterification, whereupon alkaline transesterification occurs for the unmodified remaining glyceridically bound fatty acids in the reaction product of the acidic-catalytic esterification process.

[0001] The present invention relates to a method for producing fattyacid esters of primary and secondary alcohols from raw, unpurified oilsor fats containing free fatty acids and slimy substances by applyingalkaline-catalytic transesterification with formation of a separableglycerol phase and with a preceding acidic-catalytic esterification offree fatty acids. In other words, the invention relates to a method ofproducing so-called bio-diesel, such as RME (rape methyl ester). Apartfrom native oils and the fats which have been obtained for the purposein question, the present invention can also make use of waste oils fromthe food industry which in addition to an increased content of freefatty acids, contain various other impurities, such as protein residues,combustion and frying residues as are contained in disposed frying oils.

[0002] In basic- or alkaline-catalytic transesterification reaction,glycerol fatty acid ester in form of mono-, di- and triesters is almostcompletely reacted with an alkaline hydroxide or an alkali alcoholate inthe presence of primary alcohols in amounts slightly higher thanstoichiometric amounts (with an excess of 10% to 50%) at an increasedtemperature of approximately 300 to 60° C., to the fatty acid esters ofthe primary or secondary short-chain alcohol (chain length from C1 toC4), while splitting off and separating glycerol.

[0003] It is known that this reaction may be accelerated by removing theseparated or precipitated glycerol from the reaction solution bytechnical measures such as centrifugation in separators.

[0004] Native oils, however, always contain a more or less high contentof free fatty acids which, in the above reaction, are not reacted tofatty acid esters and which react with the added alkaline catalyst tosoaps. Even press and solvent extraction oils and fats obtained incareful treatment, not refined and of qualitatively high grade mayinclude contents of free fatty acids of 1 wt.% to 3.5 wt.%. While directtransesterification of these oils is possible, correspondingly highercatalyst amounts will be required than in case of oils having no freefatty acids. Part of the catalyst is used up for neutralization of thefree fatty acids under formation of soaps while these soaps areprecipitated together with the glycerol being formed.

[0005] In order to reduce the required amount of catalyst and yieldlosses, direct transesterification of native oils is, for example,performed in several stages, wherein in the first stage only as muchcatalyst is added that the free fatty acids react completely to soapswhile using up the catalyst and a small excess of catalyst effects apartial transesterification with separation of glycerol. In this partialamount of glycerol, the soaps so formed will dissolve and cansubstantially be separated together with the glycerol. In the followingreaction stage, fresh catalyst is added which can no longer be destroyedby free fatty acids.

[0006] Crude pressed, or solvent extraction oils and fats, particularlyhot-pressed oils, always contain, in addition to free fatty acids, slimysubstances or gums which form e.g. oil/water, or glycerol/water andglycerol/oil emulsions, respectively which can only hardly be destroyed.These are essentially water-swellable slimy or mucilaginous substances,e. g. lecithins (phosphatides), and slimy substances not swellable inwater in form of other phosphorus-containing compounds. Thereby, cleanphase separation, for example between glycerol and formed fatty acidester is prevented. Moreover, washing of the generated fatty acid esterwith water which is necessary for removal of glycerol ester, isobstructed by said emulsions. The losses resulting from slimy substancesare hardly calculable; they increase, however, far over-proportionallythe losses resulting from free fatty acids with an increasing content offree fatty acids.

[0007] If the oils and fats are refined prior to transesterificationand, in this manner, freed from free fatty acids and slimy substances,the free fatty acids separated as soaps and the fatty acids bound in thephospholipides (phosphatides) constitute lost amounts which could formfatty acid esters. It is unavoidable, moreover, that in refining,neutral oil amounts are removed as well when separating the soaps andthe slimy substances together with said soaps and slimy substances andwill not take part in subsequent transesterification.

[0008] In order to be able to meet the prescribed limit values forbio-diesel with regard to the contents of bound and free glycerol andphosphorus, the slimy substances contained in oils may, furthermore, beseparated by relatively simple pretreatment, the so-called “degumming”.By this process, the slimy substances present in the oil in dissolvedform and, therefore, not separable by sedimentation, are transferredeither together with water (water desliming or degumming) or withdiluted aqueous acid (acidic desliming or degumming) into hydrated slimysubstances which are not soluble in oil and, therefore, separable asprecipitate. In separating the substances, however, neutral oil isalways entrained as well and, therefore, will not take part in thesubsequent transesterification.

[0009] If the content of free fatty acids in the used oil or fat amountsto a content of more than 2.5 or even up to approximately 5 wt.%, directalkaline transesterification, as experience shows, can no longer beperformed at reasonable cost. In case of still higher contents of freefatty acids in the oil (FFA >10 wt %), even preceding deacidificationwill lead to hardly acceptable losses. Waste fats and waste oils asobtained in the oil separators of canteen kitchens often contain suchhigh FFA contents.

[0010] Acidic catalytic esterification, or transesterification,respectively, which might alternatively be applied here, however, takesplace substantially slower than alkaline transesterification and,regarding process engineering, is generally much more elaborate. Aprocess is known (DE 196 20 523 C1) wherein, first, the oil containingfree fatty acids is treated with acidic glycerol in order to transformthe free fatty acids into mono-, di- and triglycerides.

[0011] As shown in the following, numerous attempts have continuouslybeen made since the forties to overcome the problem of free fatty acidsbeing present in general and with regard to a subsequent base-catalyzedtransesterification. While said methods have partly been quitesuccessful, the inventors of the present application had to note thatthe phospholipid problem (sometimes also referred to as phosphateproblem) cannot, thereby, be overcome.

[0012] In U.S. Pat. No. 2,415,140 A, treatment of the crude oil (havingfor example a content of 1.6% of free fatty acids) is effected by meansof a raffinate solution of glycerol dissolved in NaOH whereupon the oilphase freed in this manner from free fatty acids is centrifugallyseparated.

[0013] In U.S. Pat. No. 2,383,601 A, oils having very large amounts offree fatty acids (between 10 and 50%) are treated so thatalkaline-catalyzed transesterification becomes possible. To this end,acidic-catalyzed preesterification of the free fatty acids with methanoland concentrated sulphuric acid at increased temperature is performedprior to the transesterification. The alcohol is preferably addedover-stoichiometrically at an excess of at least more than 50% relativeto the stoichiometric amount for transesterification of free fattyacids. In one embodiment of the process, the mixture pre-esterified at60° C. is washed with water, is dried over sodium sulphate and isfiltered before alkaline transesterification takes place. In oneembodiment of the process where acidic preesterification and alkalinetransesterification take place immediately one after the other, ethanoland sodium ethylate are added directly after preesterification and thereaction mixture is transesterified at 130° C. and under pressure inorder to keep the ethanol in liquid form. After distilling off excessiveethanol, the reaction product is transferred into an acidification tank,is dried and distilled. Without such additional acidification, it is notpossible in this case to separate a glycerol phase. It isdisadvantageous that, in this kind of method, washing with water, dryingand distillation are necessary to obtain the fatty acid ester. Indistillation, amounts of free fatty acids are unavoidably also distilleddue to acidifying the glycerol phase.

[0014] In U.S. Pat. No. 4,164,506, large amounts of methanol bothserving for preesterification of free fatty acids and as entrainer(carrier) are added. After the preesterification reaction, the oil phasefreed from the free fatty acids is separated from the alcohol phasewhich contains impurities solved therein and the acidic catalyst. Asufficient separation effect can, however, only be obtained ifconsiderable amounts of methanol are used. The purified oil phaseobtained in this manner is subjected to an alkaline-catalyzedtransesterification. DE 33 19 590 A uses an entraining medium in anacid-catalytic esterification of free fatty acids as well.

[0015] In order to be able to reduce the alcohol amount, glycerol wasused as entrainer in EP 0 127 104 B1, and acidic preesterification ofthe free fatty acids was performed in the presence of water-free acidicglycerol and in the presence of alcohol. The purpose of addingwater-free acidic glycerol is to provide a catalyst and to bind reactionwater that has formed and to remove it on being separated (entrainer).In this case, too, the glycerol may, together with the acidic catalyst,be cycled in a circulation process after having removed the alcohol andthe reaction water by distillation.

[0016] The recovered alcohol, however, can be reused only after havingremoved the water. Since of the short-chain alcohols only methanol doesnot form an azeotrope with water, simple, distillative separation of thewater is possible only with methanol.

[0017] In EP 0 192 035 B1, it is set out that said method implies acatalyst separation which is relatively difficult and in any case shouldcarefully and completely be conducted while water is simultaneouslyremoved by washing the pre-esterified oil with methanol. The process asstated, moreover, implies losses of esters of the free fatty acids.Therefore, EP 0 192 035 B1 suggests as an alternative the use of solidcation exchange resins in acidic form while in this case the reactionwater should be removed after the separation of the reaction mixturefrom the exchange resin. For acidic pre-esterification, DE 42 28 476 Aworks with a strongly acidic ion exchanger in a fixed bed reactor.

[0018] In DE 43 01 686 C, the ester phase is washed aftertransesterification with glycerol, crude glycerol or glycerol phase froma preceding transesterification stage in order to avoid washing andprocess water, and transesterification takes place in two stages. U.S.Pat. No. 6,013,817 A discloses an elaborate multi-stage alkalinetransesterification process wherein glycerol phase, too, is added to thetransesterification product before the glycerol and ester phases areseparated. Distilled-off alcohol is fed back to the process. Theseparated glycerol phase is neutralized with acid, and an organic phasewith fatty acids and esters and again a glycerol phase which issubsequently used for transesterification of this organic phase areseparated.

[0019] In EP 0 131 991 A and WO 00 75098 A, on the other hand, thealkaline glycerol phase from a transesterification already performed isused for pre-treatment of a crude oil in order to extract the free fattyacids from the starting oil and to separate them together with otherharmful accompanying substances such as phosphatides from the startingoil. In this case, the fatty acids are neutralized by using the catalysthaving been already employed for transesterification.

[0020] The inventors of the present application, however, recognizedthat while the free fatty acids can be removed in an economic manner bythis procedure, a higher amount of phosphatide or phosphorus,respectively, in general, cannot be lowered below the limit valuepermissible for bio-diesel.

[0021] It is the object of the present invention to provide a method ofthe kind as referred to at the beginning being as simple andcost-efficient as possible by which method it becomes possible to lowerthe phosphorus content of the crude starting oils and fats to a requiredextent.

[0022] This problem is solved by the subject matter of claim 1.Advantageous further developments are defined in the subclaims.

[0023] In the following explanation of the present invention, referenceis made to the drawings, wherein

[0024]FIG. 1 shows a possible two-stage alkaline transesterificationwhich is applicable in the present invention,

[0025]FIG. 2 shows an acidic pre-esterification according to anembodiment of the invention, wherein washing of the reaction product ofthe acidic pre-esterification takes place in accordance with theinvention, and

[0026]FIG. 3 shows a modification of the pre-esterification withrecycling of fatty acids (split fatty acids) which are liberated orfreed from the soaps present in the glycerol by an acid.

[0027] In accordance with the invention, it is possible to provide asimple and cost-efficient process wherein, despite presence of slimysubstances (phospholipides or lecithins, respectively) in the startingoil, the free fatty acids as well can be converted into fatty acidesters, without any loss of neutral oil, and nearly quantitatively.

[0028] The process does not require any special separation of reactionwater originating in the pre-esterification of the free fatty acids andavoids employing an entrainer or carrier. The process does not fail evenif starting from lower-quality oils containing amounts of free fattyacids up to about 20 wt.% and slimy substances which otherwise could notbe transesterified at reasonable costs into useful bio-diesel containingthe permissible phosphorus amount.

[0029] To this end, the starting substance (starting oil or startingfat) is mixed with a relatively small amount being at least thestoichiometrically required amount of water-free or dehydrated alcohol(primary and/or secondary alcohol) as is required for completeesterification of the free fatty acids contained in the startingsubstance. In this connection, the alcohol excess in comparison to thestoichiometrically required amount for reacting the free fatty acidspreferably corresponds to a maximum excess value of 50%. Hence, evenwith a content of free fatty acids of more than 20 wt.%, the addedalcohol amount is still below 10 wt.% relative to the starting oil, orfat, respectively. This value is still significantly below the valueemployed in the above-mentioned use of glycerol, or methanol,respectively, as entrainer.

[0030] In U.S. Pat. No. 4,164,506, for example, alcohol amounts whichare always distinctly above the solubility of alcohol in fat are alreadyused for acidic pre-esterification. In methanol, this solubility isbetween 12 and 15 wt.% at approximately 50° C. According to U.S. Pat.No. 4,164,506, moreover, alcohol is preferably added in amounts whichare considerably increased relative to the solubility amount, namely inamounts of 20 to 30 wt.% of alcohol relative to the fat, and even moremethanol is added as entrainer for the water generated during thereaction. Finally, prior to transesterification, the alcohol phase isseparated from the oil phase purified in this manner together with theacid catalyst and the separated impurities. Due to the high methanolcontent, the alcohol phase can always be separated as an upper phase.

[0031] In the present application, on the other hand, impurities areremoved together with the glycerol phase, either with the glycerol washphase or, at the latest, together with the glycerol phase of thetransesterification product.

[0032] In this connection, it should be noted that, in the presentinvention, in case of an increased content of free fatty acids, a heavyalcohol-containing catalyst phase, i.e. lower catalyst phase can beformed in the acid pre-esterification process.

[0033] In accordance with the invention, a strongly acidic, hygroscopiccatalyst such as concentrated sulphuric acid, p-toluene sulphonic acidor the like is added at an amount which is customary when set inrelation to the oil, which is, however, in relation to the added alcoholamount, added at an increased amount of 100% maximum relative to theadded alcohol, and preferably 5 wt.% to 25 wt.% relative to the addedalcohol. Relative to the free fatty acids contained in the employed oilor fat, the acid catalyst is preferably added at a weight ratio ofcatalyst to free fatty acids of 0.1 up to 1. The free fatty acids aresubsequently reacted, acidicly catalyzed, to esters, at increasedtemperature, for example at the boiling temperature of alcohol. Inprinciple, a moderate temperature range of between 30° C. and theboiling temperature of the respective alcohol (in the case of methanolapproximately 70° C.) is recommended to obtain a separable glycerolphase. In this connection, it should be considered that the period oftime required for pre-esterification at lower temperature is generallyconsiderably high (compare for instance U.S. Pat. No. 2,383,601 A). Inaccordance with the invention, it is furthermore preferred to work atnormal pressure, where when working at an increased temperature, between1 and 4 hours were required for pre-esterification. Under the givenconditions, esterification is continued as long until a sample severaltimes washed with water to remove the added acidic catalyst shows anacid number of <1.5 (content of free fatty acids <1 wt.%). Depending onthe content of free fatty acids and the reaction temperature, thereaction time amounts to between half an hour and three hours. Afterexpiration of this time, no free fatty acids can be found in the fattyacid ester/oil mixture, not even by thin-film chromatography. Neitherthe content of mono- and diglycerides has been significantly increased.Free glycerol was not generated.

[0034] With the reaction sequence shown it is ensured that theglyceridically bound fatty acids present in the starting substanceremain practically unchanged. By simultaneous addition of alcohol andacidic catalyst, integrity of the glyceridically bound fatty acids isobviously guaranteed up to the boiling point of alcohol at normalpressure. Neither occurs an addition reaction of the acid to the doublebonds of unsaturated fatty acids which are always present in oils, inthe presence of alcohol, at least not in an extent that would disturbthe reaction and lower the yield.

[0035] It is surprising that by acidic pretreatment of the raw oil orfat in acidic-catalyzed esterification of the free fatty acids, theemulsion-forming property of the slimy substances present in the oil isdestroyed, at least, however, reduced to a degree such that it isinsignificant. During or after acidic esterification of the free fattyacids, the slimy substances may, at least partly, for example whencooling down the reaction mixture to room temperature, precipitate inform of flocs being separable for example by filtration.

[0036] The method of the invention implies a processing or workingwindow wherein the water formed in the esterification of the free fattyacids is withdrawn by the added hygroscopic acidic catalyst, for exampleby hydration, from the esterification reaction, and is neither strippednor carried or entrained by methanol. At the same time, the ability ofthe catalyst to chemically bind to the double bonds of unsaturated fattyacids is destroyed. However, the characteristic of the acidic catalystbeing probably associated with reaction water and/or alcohol, of beingable to destroy the emulsion-forming property of slimy substances issufficiently preserved. It seems that by the method of the invention,even the fatty acids bound in the slimy substances are reacted to fattyacid esters. The phosphorus bound therein is removed with the glycerolwash phase and, at the latest, with the glycerol formed in thetransesterification stage.

[0037] Carried out investigations of the ester phase have shown that itis free from phosphorus-containing compounds while the glycerol phase,on the other hand, contains phosphorus in dissolved form (which wasdetectable after corresponding pretreatment, for example after ashing inthe presence of magnesium oxide).

[0038] In FIG. 2, there are provided for pre-esterification a mixingstage and three reaction vessels which can be raised to the requiredincreased temperature and which include a heat exchanger. This isadvantageous in that, while pre-esterification is performed batch-wisein the three reaction vessels, washing by the decanter can be performedcontinuously and at a high throughput.

[0039] The glycerol phase formed in an alkaline transesterficationhaving already been carried out is used, according to the invention, forneutralization of the reaction product of the acidic esterificationstage. In the prior art, to the contrary, this glycerol phase isemployed for washing the starting oil or for washing the reactionproduct of transesterification.

[0040] In accordance with the present invention, therefore, it is onlyafter termination of the acidic esterification reaction that theobtained reaction mixture is washed prior to alkalinetransesterification with the alkaline glycerol phase originating from aprevious alkaline transesterification (FIG. 1). It is not necessary tocarefully separate the ester originating from the alkalinetransesterification from the glycerol phase since the ester remains inthe reaction mixture. The separation of the glycerol wash phase may thenbe carried out by decantation or, more advantageously, by centrifugationsince the glycerol phase is not soluble in the ester phase. In thiscase, too, residual amounts of wash glycerol contained in the neutraloil/ester phase do not disturb. The lower heavy glycerol phase to bedischarged, however, should as much as possible be free from ester andoil. To this purpose, an extraction decanter is preferably used, i.e. asolid wall bowl centrifuge, in which the glycerol phase may particularlywell be separated from the remaining liquid phase (FIG. 2). The decanterwhich is charged with the alkaline glycerol phase and the reactionproduct of the acidic pre-esterification, serves simultaneously aswashing and separation means so that the process, despite theintermediate provision of the washing step, can very effectively beperformed.

[0041] Since the alkaline glycerol phase contains practically thecomplete amount of alkaline catalyst, the complete amount of basic oralkaline catalyst can be used, after alkaline transesterification, forneutralization. The totally required alkaline catalyst amount may inthis manner be considerably reduced. Moreover, the glycerol amountoriginating from transesterification may, at least partly, bind andremove reaction water of the acidic esterification. By suitablyselecting the ratio of acidic to alkaline catalyst, the pH value ofdischarged glycerol phase can be controlled and thus glycerol can moreeasily be obtained. To this end, formation of a basic pH value will as arule be preferred and the discharged glycerol phase will correspondinglybe adjusted.

[0042] In accordance with the invention, the glycerol phase from thesubsequent transesterification may be used without conditioning in awashing step and can again be discharged without clean separation.Residues of the glycerol wash phase will not disturb the subsequentalkaline transesterification. This is in complete contrast to theglycerol entrainer in the prior art mentioned above since in that case,the catalyst-containing entrainer phase had to be reliably separated. Inorder to make clean separation possible, relatively large methanolamounts have been necessary.

[0043] During said, otherwise uncritical, separation of the glycerolwash phase prior to transesterification, care should be taken that noesters are removed as well.

[0044] The intermediate product in form of the washed mixture of fattyacid glycerides which have remained unmodified and of the formed fattyacid esters is subsequently alkaline-catalyzedly transesterified. Tothis end, an alkaline catalyst, for example alkali hydroxide or analcoholate, preferably of the alcohol used, is directly added. Thealkaline catalyst causes alkaline transesterification, i.e.transesterification of the glyceridically bound fatty acids into fattyacid esters. The esters having been formed in the acidic process stagefrom the free fatty acids do not participate in the reaction.

[0045] The water formed in the acidically catalyzed esterification ofthe free fatty acids and having not completely been separated by washingdoes not interfere with the alkaline-catalyzed transesterification ofthe fatty acid tri-, di- and mono-glycerides into fatty acid esters ofthe primary (and secondary) alcohols.

[0046] Alkaline transesterification is performed according to FIG. 1 intwo stages, wherein, after the first transesterification stage, themethyl ester and mono and di-glycerides still present are separated fromalkaline glycerol by a separating decanter (solid wall bowl centrifuge).Separation into methyl ester and alkaline glycerol is performed afterthe second transesterification stage, for example in a centrifugalseparator. In the drawing, furthermore, the required mixing stages areshown.

[0047] The fatty acid ester mixture formed according to the method ofthe invention may, advantageously after having destroyed the alkalinecatalyst by neutralization, for example, by adding stoichiometric acidamounts, or by adding corresponding amounts of an acidic notester-soluble adsorbent, for example in form of acidic bleaching clays,be purified from the excess amounts of alcohol, possibly present, bydistillating-off the alcohol and by filtration in order to separate theadded adsorbent. By this purifying operation, residual amounts ofglycerol, if present, and solved in crude ester are removed as well.

[0048] A different, or additional, manner of purifying the raw estermixture consists in washing the mixture with water. In this manner, too,the residual amounts of salts, catalyst, alcohol and glycerol cancompletely be removed. A disturbing formation of emulsions was notobserved when carrying out esterification according to the invention.

[0049] For isolation of the fatty acid esters, the raw fatty acid estersas such can be subjected to distillation (preferably vacuumdistillation). Thereby, they can be freed from waxes and otherester-soluble oil or fat- accompanying substances.

[0050] However, for use as bio-diesel this is generally not necessary.

[0051] By means of the method of the invention it was possible todrastically lower the phosphorus content in the esters obtained. Aphosphorus content of 175 mg/kg of the starting oil was, for example,lowered to less than 1 mg/kg in the bio-diesel produced.

[0052] In the method of the invention, the fatty acid ester amountobtainable relative to the fatty acids bound in the oil subjected to themethod and relative to the content of free fatty acids contained in theoil, is nearly quantitative.

[0053] An advantageous further development of the method of theinvention (FIG. 3) consists in acidifying the reaction glycerol used aswashing glycerol after the washing step and, in this manner, to liberatethe fatty acids from the soaps contained in the glycerol and to separatethem together with entrained neutral oils. This is preferably effectedby using a centrifuge in which a separable liquid light upper phasewhich contains fatty acids as well as entrained neutral oil, isobtained. In FIG. 3, sulphuric acid is used for acidifying the glycerolsoap phase separated in the extraction decanter, which, to this end, isfed to a mixer. In a separator, the light “upper phase” is separatedfrom the acidic raw glycerol and is directly returned into the acidicpre-esterification of the oil. In this manner, it becomes possible toconvert the fatty acids which, due to partial saponification, arepresent in form of soaps and are discharged together with the glycerolphase from the alkaline transesterification process, as well as neutraloils into methyl ester and, in other words, thus raising the yield to amaximum possible value.

EXAMPLE:

[0054] Preparation of the Starting Oil

[0055] A. Soapstock obtained from a refining process of native sunflower press oil and a same amount of slimy substances separated duringrefining were reacted with aqueous mineral acid in common manner tosplit fatty acid, were separated from the acidic aqueous phasecontaining excess amounts of mineral acid, were mixed with the sameamount of slimy substance phase as separated in the refining, and themixture of split fatty acids, slimy substances and entrained neutral oilso obtained was dried.

[0056] B. 95 g of native High-Oleic sun flower oil having an acid numberof 3 were reacted with 10.5 g of the mixture produced sub A. The mixtureso produced had an acid number of >10.

[0057] Applying the Method

[0058] a. 500 g of said mixture were heated to 58° C. and were mixedwith a mixture of 2.1 g of concentrated sulphuric acid and 10 g ofmethanol (technical, water-free). After a few minutes, further 35.5 g ofmethanol were added. By using altogether 45.5 g of methanol for theacidic esterification, less methanol was added than is required forreacting all free and bound fatty acids into fatty acid ester. Samplesdrawn at various time intervals showed a rapidly decreasing acid number.After a reaction time of 2 hours, the acid number had decreased to 1.5corresponding to an FFA (free fatty acid) content of <1 wt.%.

[0059] To estimate the performed process with regard to the behavior ofthe glycerides, further 25.7 g of methanol were added and it was stirredfor a further hour at 58° C. With 71.2 g, the reaction mixture nowcontained more methanol than is required for reacting all free and boundfatty acids to fatty acid esters. Precipitation of glycerol could not beobserved.

[0060] A glycerol phase separated in an already-performed alkalinetransesterification process was added to the reaction solution generatedsub step a. while intensively mixing and in such an amount that a samplein an aqueous extract indicated a pH value of approximately 7.Altogether 36.7 g of alkaline glycerol phase were required. The washglycerol was separated in a separating funnel as heavy phase from theester phase and drawn off. The wash glycerol, when diluted with water,showed neither emulsion tendency nor was there a precipitate in form ofa lighter organic upper phase. It was subsequently free from fatty acidesters and from slimy substances being not soluble in basic aqueousphase.

[0061] b. The oil/ester phase separated from the wash glycerol phase wasmixed with 17 g of 30 wt.% sodium methylate/methanol solution andmaintained under stirring for 30 minutes at approximately 60° C.Subsequently, the reaction mixture was transferred into a separatingfunnel for separating the glycerol formed. 104 g of alkaline glycerolphase were separated. This glycerol phase could directly be used infurther process executions for washing the acidic esterification phaseof step a. The thin-layer chromatogram of the ester phase did notexhibit any mono-, di- and/or tri-glycerides and showed only fatty acidmethyl ester.

[0062] In experiments in our Pilot Plant, the present and furtherexamples were reproduced, using correspondingly larger amounts and thedevices schematically shown in the figures. Crude High-Oleic sun floweroils, normal sun flower oil and rape oil having acid numbers of over 3up to about 40 (corresponding to an FFA content of 20), have, forexample, been successfully transesterified while simultaneouslyeffecting desliming.

1. Method of producing fatty acid esters of primary and/or secondaryalcohols from raw unpurified oils and fats containing free fatty acidsand slimy substances, or from waste oils loaded with free fatty acidsfrom the food industry using alkaline catalyzed transesterificationunder formation of a separable glycerol phase and an acidic catalyzedesterification of free fatty acids preceding the transesterification,characterized in that a. first the acidic catalyzed esterification ofthe free fatty acids together with most extensive deactivation of slimysubstances being present is performed with regard to the disadvantageousemulsion-forming properties thereof in that a water-free primary and/orsecondary alcohol and a strongly acidic hygroscopic catalyst are addedto the starting substance in form of the raw oil or fat or waste oil,respectively, wherein the added amount of water-free alcohol at leastcorresponds to the amount being stoichiometrically necessary forcomplete ester formation of the free fatty acids contained in thestarting substance, b. that the reaction product of the acidic-catalyzedesterification is washed with the separated glycerol of an alkalinetransesterification process having already been performed and theglycerol wash phase is subsequently separated, c. that the reactionproduct of the acidic-catalyzed esterification, having been washed inthis manner, is subjected to said alkaline-catalyzed transesterificationin that an alkaline catalyst and the amount of alcohol required fortransesterification of the glyceridically bound fatty acids into fattyacid esters are added to said reaction product, and d. that the glycerolphase formed in step c. is separated from the esters of the free fattyacids and of the fatty acids present in glyceridically-bound form in thestarting substance, having been formed in steps a. and c.
 2. Methodaccording to claim 1, characterized in that an alcohol excess relativeto the stoichiometric amount in step a. amounts to 50% maximum, and thatthe acidic catalyst is added with 100 wt.% maximum, preferably 5 to 25wt.%, relative to the added alcohol.
 3. Method according to claim 1 or2, characterized in that for, or during course of, the acidic-catalyzedesterification, no entrainer is added.
 4. Method according to one of theforegoing claims, characterized in that step a. of the esterification ofthe free fatty acids is performed at increased temperature so that theglyceridically-bound fatty acids according to step a. are still presentin unmodified form.
 5. Method according to one of the foregoing claims,characterized in that when adding alcohol in step c., an alcohol excessis limited to 50% in relation to the stoichiometric amount fortransesterification of the bound fatty esters.
 6. Method according toone of the foregoing claims, characterized in that the water-freealcohol is selected from short-chain alcohols, methanol, ethanol,propanol, isopropanol, butanol and/or isobutanol.
 8. Method according toone of the foregoing claims, characterized in that separation of theglycerol wash phase is performed under centrifugation, preferably bymeans of a continuously operating centrifuge, particularly an extractiondecanter.
 9. Method according to one of the foregoing claims,characterized in that water-free alcohol is added with an excess amountof 5% to 40% relative to the stoichiometrically required amountaccording to step a.
 10. Method according to one of the foregoingclaims, characterized in that concentrated sulphuric acid or p-toluenesulfonic acid is used as the acidic catalyst.
 11. Method according toone of the foregoing claims, characterized in that the acidic catalystis added in a weight ratio of catalyst in relation to free fatty acidscontained in the oil or fat employed, ranging from 0.1 up to
 1. 12.Method according to one of the foregoing claims, characterized in thatthe glycerol phase separated in step d. is acidified and subsequentlythe acidic raw glycerol is centrifugally separated from a lighter phasestill containing residual free fatty acids and neutral oil, whichlighter phase is returned into the acidic pre-esterification stage ofstep a.